1. Technical Field
The invention relates to a process for recovering oil from a subterranean oil-bearing formation and more particularly to a process for cycling wettability-altering fluids through a subterranean oil-bearing formation to enhance oil recovery therefrom.
2. Background Information
The art recognizes that oil recovery by means of conventional waterflooding is ineffective in fractured formations because channeling occurs through the fracture network. The art proposes a number of solutions to this problem. Brownscombe et al, Stone et al, and Sengul et al expressly employ imbibition displacement processes to increase oil recovery relative to conventional waterflooding in fractured formations.
"Water-Imbibition Displacement", E. R. Brownscombe et al, Oil & Gas Journal, v. 51, n. 28, Nov. 17, 1952, page 264, describes an oil recovery process employing water-imbibition displacement in the naturally-fractured water-wet Spraberry Field of West Texas.
U.S. Pat. No. 3,498,378 to Stone et al utilizes a surfactant solution in the Spraberry Field to pressurize the formation and change its preferential wettability from water- to oil-wet. Oil is then produced until the water-to-oil ratio reaches an economic limit. Production is interrupted and water is injected into the formation to restore the bottom hole pressure. Oil production resumes again until it falls below acceptable economic limits. Pressure pulsing with water is repeated until diminished recovery necessitates the injection of additional surfactant. The cycle can be repeated until the formation is depleted.
U.S. Pat. No. 4,694,904 to Sengul et al cyclically floods a fractured formation with two separate fluids having disparate mobilities. The lower mobility fluid may be a polymer solution and the higher mobility fluid may be water or a surfactant solution. The process is designed to match the performance of a continuous polymer flood, but at a lower cost because polymer use for the process is reduced.
U.S. Pat. No. 4,364,431 to Saidi et al utilizes a surfactant to augment a waterflood which displaces oil from a fractured oil-wet formation by a gravity drive mechanism rather than an imbibition displacement mechanism. Saidi suggests that the surfactant reduces the interfacial tension between the water in the fractures and the oil in the matrix blocks of the formation, which enables the oil to enter the fractures where it is driven upward to a producing well by the density difference between water and oil.
The above-cited references fail to realize the full potential of an imbibition displacement process to recover oil from a formation. Although Brownscombe et al, Stone et al, and Sengul et al expressly employ an imbibition displacement mechanism, none of the references recognize specific improvements which could render the mechanism more effective. Furthermore, none of the references recognize the utility of the mechanism beyond water-wet formations.
A need exists for a process which substantially improves the performance of fluids which are injected into a fractured formation to recover oil from the formation. A further need exists for a process which both maximizes the amount of oil recovered from a fractured formation and accelerates the rate of oil production from the formation without substantially increasing the volume requirement of injected fluids.